A conventional process for producing a zinc oxide varistor is disclosed, for example, in Japanese Laid-open Patent No. 61-259502, and its procedure is as follows.
First, ZnO is a principal ingredient, and small amounts of metal oxides such as Bi.sub.2 O.sub.3, Co.sub.2 O.sub.3, MnO, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, NiO, and Al.sub.2 O.sub.3 are added as auxiliary ingredients. Mixing them sufficiently together with water, binder, and dispersant, slurry is prepared, which is dried and granulated by a spray dryer, and the obtained power is formed in a disk of 55 mm in diameter and 30 mm in thickness. After baking at 500.degree. C. in order to remove organic matter, it is calcined at 1020.degree. C., and a calcined material is obtained. A prepared slurry for forming a high-resistance layer is applied on this calcined material by means of a spray gun.
This slurry for forming a high-resistance layer is prepared by reacting Fe.sub.2 O.sub.3, ZnO and Sb.sub.2 O, to produce ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12, weighing powder of ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12 so that the ratio of Fe and Sb may be 2:1, adding purified water so that the ratio by weight to this powder may be 1:1, and adding binder such as polyvinyl alcohol for increasing the strength of the coat film by about 0.1 wt.%.
Consequently, the calcined material on which the slurry for forming a high-resistance layer is applied is baked in air at 1200.degree. C. to obtain sinter, and both ends of the sinter is polished to form an Al sprayed electrode, thereby obtaining a zinc oxide varistor having a lateral high-resistance layer.
In this conventional method, as the slurry for forming a high-resistance layer, ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12 preliminarily synthesized at high temperature are used, and when a lateral high-resistance layer is formed by using them, the reactivity of the calcined material with ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12 is not sufficient, and the contact between the sinter and the lateral high-resistance layer is poor, and the lateral high-resistance layer is likely to be peeled off during discharge current withstand test, and hence the discharge current withstand capacity characteristic is low.